The present invention relates to a method presented in the preamble of claim 1 and to an apparatus presented in the preamble of claim 10 for eliminating D.C. offset and achieving AM suppression in a direct conversion receiver.
In a direct conversion receiver, radio frequency signals are converted directly into baseband signals, whereby separate intermediate frequency stages are not required. In that case, the number of high frequency components needed in direct conversion receivers is smaller than in conventional receivers which include intermediate frequency stages. Due to less complexity, the integration degree of direct conversion receivers can be increased compared to receivers which include intermediate frequency stages.
However, receivers implemented with the direct conversion technique have the problem that their dynamic range is smaller than in receivers which include intermediate frequency stages. The dynamic range is adversely affected by the fact that in addition to the high frequency signal of the reception channel, the mixer of the receiver also receives high frequency signals of the adjacent channels, whereby due to the non-ideality of the mixer, a disturbing D.C. offset is produced on the output of the mixer. Thus the stronger signals of the adjacent channels can produce a substantially higher D.C. offset in the signal than the desired signal expressed on the reception channel.
In a digital receiver, the decision on whether the transmitted symbol is 0 or 1 is made on the basis of the voltage level of the demodulated signal. Thus the D.C. offset can cause a wrong decision to be made in the receiver concerning the transmitted symbol. In an I/Q-modulated signal, two consecutive symbols are combined into I and Q signals.
Thus I and Q signals are produced from the received signal in the I/Q demodulator, on the basis of which a decision is made in the receiver as to which symbol pair (00, 01, 10, 11) has been transmitted. The D.C. offset can occur in both I and Q signals, whereby a wrong decision can be made in the receiver as to the signal pair transmitted. In the worst case, even the error correction logic of the receiver cannot correct the information that has got a faulty expression.
There are some prior art solutions, in which an attempt is made to express the signal of the reception channel in spite of a high interfering D.C. offset. However, a drawback of these solutions is the fact that they only operate in situations in which the disturbing D.C. offset is constant or changes very slowly. In situations in which the powers of the signals in the adjacent channels vary quickly, the disturbing D.C. offset also changes quickly, whereby the prior art solutions are not capable of fully eliminating the disturbance caused by the D.C. offset. This is a typical situation in TDMA systems, for example.
The published solutions to eliminate the problem caused by the D.C. offset are mainly based on various DC filter applications. When a narrowband filter is used, the settling time becomes long, whereby the filter cannot react to quick changes of power. On the other hand, with a wideband filter it is possible to achieve a short settling time, but a filter of this kind also filters a substantial part of the useful signal, whereby the performance of the receiver is reduced.
The U.S. Pat. No. 5,212,826 presents a method for eliminating the D.C. offset in a manner such that, during the delay period of the receiver, the HF signal is prevented from entering the receiver, and the D.C. offset thus appearing in the receiver is measured. On the basis of the measurement, a constant correction voltage is produced and fed into the correction circuit of the receiver during the reception stage. A new measurement is performed during the next delay period, and a correction voltage corresponding to the new measured value is fed to the correction circuit during reception. A drawback of this method is, for example, the fact that the correction voltage is constant during the whole reception phase, and a HF signal is not fed to the receiver during the measurement, whereby the D.C. offset caused by the HF signal cannot be eliminated.
The British patent application GB 2 274 759 presents a method in which the D.C. offset is assessed by filtering from the demodulated baseband signal. This method can be applied primarily in cases in which the change of the offset voltage is substantially slower than the bit rate of the received signal.
In the prior art solutions, the correction of D.C. offset is based on a corrupted signal and possibly some advance information of the useful signal, but the source of the interference is not taken into account in the correction.